[Info-vax] BASIC, Performance, and Stack Walking

[Dave Froble]

On 12/17/2018 9:14 AM, John Reagan wrote:
> On Sunday, December 16, 2018 at 10:35:34 PM UTC-5, Stephen Hoffman wrote:
>> On 2018-12-17 02:25:15 +0000, Tim Sneddon said:
>>
>>> Part of the BASIC prologue requires the routine to hunt for some sort
>>> of context.
>>
>> Some of the previous discussions...
>> https://groups.google.com/d/msg/comp.os.vms/OVTUn9GDHPo/tdohHkGo2nQJ
>> https://groups.google.com/d/msg/comp.os.vms/a58LJeU8FvQ/5jLe-0-6BQAJ
>>
>> That DBASIC$IO_NO_SIGNAL logical name means the C feature logical name
>> design morass has infested BASIC, too.
>>
>> --
>> Pure Personal Opinion | HoffmanLabs LLC
>

> I was hoping nobody would mention that as I hope that nobody uses it
> and I can remove it.
>
> Here's the story:
>
> #1  Every BASIC routine calls into the RTL in the routine prologue to
> walk up the stack looking for any active BASIC frames to verify that
> the scale factor is identical.  Apparently, if they aren't, a hole
> opens in the space-time continuum.

I've never used that scale factor, and never heard of anyone doing so. 
But I don't get out much.

>  This stack walk is cheap on Alpha
> but expensive on Itanium.  The RTL was updated for Itanium to do this
> as quick as possible, but it still hurts on Itanium.  You can turn
> off that RTL call with OPTION INACTIVE=SETUP but several different
> BASIC language features will turn it back on (see the BASIC User
> Manual).  We are thinking to simplify the RTL's per-routine overhead
> and not to do that check by default.   BASIC already turns on
> checking by default (ie, /CHECK=(BOUNDS,OVERFLOW).  I want to add an
> off-by-default /CHECK=SCALE_FACTOR setting.

How much would this help?  Sure sounds like a great idea to me.  Might 
affect someone else.

> #2  The BASIC RTL will signal RMS errors, like file-not-found, on IO
> statements.  Programs that normally rely on such signals as part of
> their normal operation noticed slower operation on Itanium.  Unlike
> Pascal which as "ERROR := CONTINUE" and "ERROR := MESSAGE" on all IO
> routines, BASIC gives you the single behavior of signalling and
> expects you to catch that will a handler (many people use
> LIB$SIG_TO_RET).  So we added support to the RTL to allow you to set
> a "do not signal, but return instead" on a "per-call" basis and a
> "per-lun" basis.  Subsequently (from reading the notes and comments),
> HPE India added a BASIC feature logical to change the default.  So
> there must be at least one customer who found it difficult to modify
> their application or didn't have sources to the application.  I did
> an eye-roll when I saw that edit to the RTL when I returned to VSI
> and reviewed the changes.  I want to remove that logical for x86
> since the unwinding/signalling overhead on x86 is much smaller (not
> quite as small as Alpha, but much smaller than Itanium).
>

Thanks John, any help is greatly appreciated.

On this susbject, I find myself 110% in agreement with Steve's "break 
backward compatibility" ideas.

-- 
David Froble                       Tel: 724-529-0450
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